The rated range is determined by following a scripted driving regimen that few people would ever follow and it only does a small portion of higher velocity running. Plus, it's all done on a dynamometer, so you don't have the aerodynamic factor thrown in.
The community average for efficiency (if you believe BMW's app) is around 4mi/kwhr. 2.9 is quite a bit off. Some people have reported as much as nearly 7, the best I've ever seen on mine was about 6.5, but average closer to that 4 the community reports. 75-80 mph is going to suck up energy. There's a squared factor in there, so the difference is not linear. 80*80/60*60=1.777x more aerodynamic drag at 80 verses 60. Speed does matter a lot. The i3 isn't bad with a cd factor, but the i3s is worse, making the effect more of a factor. Wider tires affect not only the aerodynamic drag, but also the rolling drag and there are other things on the i3s that affect it as well. Can't have your cake and eat it, too.
Bottom line, if you need to go further on a single charge...slow down.
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)